The invention relates generally to the field of numerically controlled turning machines; and specifically, the invention provides a numerically controlled turning machine for automatically cutting threads on a rotating workpiece in response to an input program defining the parameters of the thread cutting cycle.
Typically, on a thread cutting numerically controlled turning machine, a transducer is connected to the rotating spindle holding a workpiece; and a spindle signal is generated therefrom which represents the angular velocity of the spindle. The numerical control uses the spindle signal in conjunction with a programmed input signal defining the thread lead to generate command pulses to a servomechanism circuit for controlling the motion of the cutting tool relative to the rotating workpiece.
In cutting a thread, the numerical control causes a single point cutting tool to move iteratively through a number of thread cutting passes over the rotating workpiece. The depth of cut is accumulated with each pass; and when the final thread depth is achieved, the cycle of iterative motion is terminated. Once every revolution, an index or start pulse is generated from the spindle signal. With each cutting pass, the cutting tool is moved to a start point; and the index pulse is used to synchronize motion of the cutting tool relative to the rotating workpiece.
In earlier prior art systems, each threading pass had to be individually programmed which was a difficult and time consuming process producing excessive lengths of program tape. With the advent of computer numerical control the numerical controls have simplified much of the programming work. This is especially true for machining operations which are repetitive in nature.
For example, British Pat. No. 1,438,163 discloses an apparatus which responds to a tape program containing the final configuration of a part and generates a number of contouring cycles therefrom. The control contains a final configuration output unit, a contouring cycle instruction unit, an intermediate contouring position instruction unit and a judging and selection unit to determine the final size. A fixed general cycle of operation is built into the control. By means of the final configuration parameters and the constant depth of cut, the program places limits on the general cycle of operation. The control generates a number of iterative machining cycles until the final workpiece contour is achieved.
U.S. Pat. Nos. 3,854,353 and 4,017,723 disclose a method and apparatus for automatically generating iterative thread cutting cycles in response to programmed-information defining the thread cutting path parameters and the thread cutting depth. Given this information, the control generates a number of thread cutting passes at the programmed constant depth increment.
There are a number of disadvantages to the prior art devices which the present invention overcomes. First, in the prior art, successive passes are generated at a constant depth of cut. As is appreciated by those who are skilled in the art, the thread cutting tool for V-shaped thread is a relatively delicate triangular shaped tool. During the initial thread cutting passes, only a small amount of material is being removed; therefore, the depth of cut may be relatively large. However, with subsequent passes, greater amounts of material are being removed; and the depth of cut must be decreased to prevent tool breakage. The magnitude of the constant depth increment in the prior art devices must be compromised to the worst case situation. Therefore, during the initial passes, the cutting tool is not working to its potential, and inefficiency results.
Further, the programming of information in the prior art patents is somewhat burdensome. For example, U.S. Pat. No. 3,854,353 three blocks of information must be programmed to define a thread. With the present system, a single block of informatin is used in conjunction with parenthetical expressions defining the less variable thread parameters. Once the parenthetical expressions are programmed, they are used for all subsequent threads until they are specifically changed.
Further, with the device in U.S. Pat. No. 3,854,353, the thread start points are defined along a line from the start point (X.sub.0,Z.sub.0) to the point (X.sub.1,Z.sub.1) defined by the first block of tape. The relationship of these points must be calculated for each threading block. This line defined by the locus of thread start points forms an acute angle with the X machine axis. This angle is standard for many threads; and in the present invention the tangent of the angle is programmed as a constant in a parenthetical expression. Therefore, the start point need not be programmed along the line defined by the locus of thread start points.